1. Field of the Invention
The present invention relates generally to lasers, and particularly to a distributed Bragg reflector (DBR) laser for use as a transmitter in optical communications.
2. Technical Background
For a wavelength-division-multiplexed (WDM) optical network, the number of independent channels simultaneously transmitted over one physical fiber equals the number of discrete wavelengths transmitted. These channels allow system integrators to explore high information throughput, flexible bandwidth management, optical transparency and optical add/drop switching in a cost effective manner.
In a telecommunication DWDM system application, the transmitter wavelength has to be locked to one of the International Telephone Union (ITU) standard wavelengths of an ITU grid to meet crosstalk specification and ensure reliable operation of the system over its normal operating lifetime (about 25 years). For a multiwavelength optically controlled phased array (phasar) antenna application for use in a dense wavelength division multiplexing (DWDM) system, the number of antenna array elements is equal to the number of wavelengths.
It is highly desirable to have an integrated multiwavelength laser with high wavelength accuracy to support the above DWDM application. Such a laser has to satisfy the following criteria: a stable wavelength comb pre-determined by fabrication; a simplified optical packaging; component sharing (such as a common temperature cooler and optical isolator); simplified testing; and compactness. The above merits should reduce the per-wavelength transmitter cost in both initial procurement and subsequent operation. As is known, a lasing wavelength comb refers to a laser array having a number N of lasers whose output wavelengths are separated by a channel separation or wavelength spacing.
The number of channels available on a single laser chip is limited by the material gain bandwidth and wavelength spacing between the channels. In order to take advantage of the flat gain region of silica-based erbium doped fiber amplifiers (EDFAs), the system wavelengths are restricted to 1545 to 1560 nm. For a 40-channel system, the desirable wavelength spacing is 0.4 nm (or 50 GHz for frequency spacing). It is known that the lasing wavelength of a free-running commercial distributed-feedback (DFB) laser, determined by its built-in DFB grating and the refractive index of the semiconductor waveguide, changes with temperature and has to be temperature-controlled. For example, DFB lasers are often used with a temperature cooler and/or an isolator to maintain a fixed wavelength. With one known fixed DFB laser array technology, a wavelength accuracy of xc2x10.2 nm (25 GHz) within a given on-chip power combiner and a shared output semiconductor optical amplifier (SOA) can be achieved with high yields. The minimum system wavelength spacing that can be supported by this laser array is about 1.6 nm (200 GHz) since the flat top region of a multiplexer/demultiplexer filter response is a fraction of the channel spacing. Especially for ring or long-distance networks where many filter cascades are required, the wavelength spacing must be defined accurately.
Although wavelength accuracy can be significantly improved with the use of wavelength tunable laser arrays such as the DBR laser array, laser arrays with fixed wavelengths are more desirable than wavelength tunable ones from the perspective of simple operation and long-term reliability.
One approach for the realization of multiwavelength lasers is to integrate an array of gain elements with a wavelength multiplexer to form a phased-array (phasar) laser. In such a phasar laser, a multiplexer is placed inside a laser cavity that is defined by a high reflection coated facet on one cleaved edge and an opposed as-cleaved facet.
As is known, a shared gain element is optional. With a low loss multiplexer, 18-wavelength simultaneous continuous wave (CW) operation of a phasar laser has recently been demonstrated in the literature. In this case, the channel spacing of the laser equals to the channel spacing of the wavelength multiplexer. The wavelength spacing of the phasar laser is expected to be extremely uniform since the wavelength spacing fluctuation is in the order of the longitudinal mode spacing ( less than 0.02 nm). With this approach, a 40-channel combiner with 0.4 nm wavelength spacing is achievable. However, direct modulation in excess of 1 Gb/s per channel of the phasar laser has not been achievable due to the length limitation of the long laser cavity.
Long cavity lasers cannot be directly modulated at a high bit rate since the 3-dB modulation bandwidth decreases as the cavity length increases. If the peak frequency is close to one of the harmonic frequencies of the signal, the signal will be distorted. Hence, for high bit rate applications, a long cavity laser needs a high speed external modulator to shift the peak frequency back towards the center of the modulation bandwidth. This external modulator is quite expensive. Hence, there is at least a cost-saving reason to integrate a long cavity laser with an internal modulator for external modulation.
The severe optical crosstalk due to the shared output optical gain element is another drawback, which restricts simultaneous multichannel modulation.
Another phasar approach has been demonstrated in an 8-channel digitally tunable transmitter that has an electroabsorption modulated output formed by selective-area epitaxy. Although such a digitally wavelength selectable phasar laser integrated with one electroabsorption modulator was demonstrated using the second order diffracted light as the output, its output power was low (at approximately xe2x88x9216 dBm). Since this selectable phasar laser requires a small free spectral range and anti-reflection (AR)/high reflection (HR) split coatings and on one facet, this chip is large and its application of split facet coatings is difficult. Hence, there is a desire to provide a laser capable of simultaneous multi-channel modulation, in excess of 2.5 Gbits/s per wavelength while providing wavelength accuracy and selectivity, as additional features, without complications, such as split coatings.
One aspect of the present invention is the combined advantages of a multiwavelength laser formed from a laser cavity defined on a first reflective end by a phased-array multiplexer and on a second reflective end by a broadband mirror.
In another aspect, the present invention includes a distributed-Bragg-reflector as the broadband mirror.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.